What are black holes? WIRED explains

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Michael Parker from Cambridge University was studying distant galaxies to learn about a poorly understood phenomenon of black holes when he stumbled across this unique feature around the active galactic nuclei AGN IRAS 13224−3809.

AGN is a term used for the supermassive black hole found at the centre of a galaxy. Black holes are famous for pulling in anything around them, with gravity so strong that not even light can escape. At the same time, however, black holes at the centre of galaxies can spew out wide streams of gas called outflows.

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As most black holes are surrounded by other clouds of warm gases, outflows can be difficult to measure – but not for IRAS 13224−3809.

“[AGN IRAS 13224−3809] is the most X-ray variable AGN - it can change in brightness by a factor of 50 or more in around an hour, so it's a very exciting black hole to look at,” Parker told WIRED.

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“It's a particularly good source for looking at the outflow, as it has no other absorption in the system - often there's other warm or cold gas floating around that gets in the way and confuses the detections.”

Astronomers detect the amount of gas flowing out from the black hole by studying how its electrons absorb X-rays – in an X-ray absorption line. Because IRAS 13224−3809 changes brightness so quickly, astronomers can use it to test how changes in brightness affect the outflows. By studying the AGN, the researchers found the absorption lines disappear when the black hole is at its brightest.

“This is due to the X-rays ionizing the gas in the wind, stripping the electrons from the atoms so they no longer produce absorption features,” says Parker. “It's not unexpected, but it's the first time we've been able to link the two. Now that we can see the relationship, we can start to understand how the winds are formed and driven.”

The unique features of AGN IRAS 13224−3809 mean it will give us an opportunity to study outflows in a way that has never been done before.

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“Outflows from AGN are one of the mechanisms for AGN feedback, where the black hole regulates the growth of its host galaxy by driving out the interstellar medium, cutting off star formation,” says Parker.

“This means that to understand how galaxies, including our own, end up the way they are we have to understand how the physics of these winds. Our campaign on IRAS 13224 is now the best dataset for learning about this.”